The present invention relates to X-ray imaging screens utilizing phosphors disposed in microchannels disposed in a plate. More specifically this application relates to the “tiling” of such microchannel plates to form a larger imaging area and to the use of “storage phosphors” in the microchannel plates which enables the phosphors to be read out after exposure and from the side exposed to the X-rays. The storage phosphor screens of the present invention provide significantly increased resolution than the prior art storage phosphor screens.
Fine detail visualization, high-resolution high-contrast images are required for many X-ray medical imaging systems and particularly in mammography. The resolution of X-ray film/screen and digital mammography systems is currently limited to 20 line pairs/mm and 10 line pairs/mm, respectively. In particular, light scattering by the phosphor particles and their grain boundaries results in loss of spatial resolution and contrast in the image. In order to increase the resolution and contrast, scattering of the visible light must be decreased. The present invention is directed to a novel microchannel composite screen design, which provides high resolution, high contrast, and efficient X-ray to visible light conversion screens for X-ray imaging. The microchannel phosphor screen can be used in both electronic (digital) and film (analog) X-ray imaging.
A conventional X-ray screen, as shown in FIG. 1 herein has a thickness of about 30-300 microns (μm) and consists of phosphor particles with a mean size between a few to 10 microns. The light generated in the screen by the incident X-ray diffuses towards the film emulsion over the finite thickness of the screen material. As the light diffuses, it spreads out which results in a loss of spatial resolution and contrast in the image. To improve resolution and contrast, thinner screens could be employed. However, use of the standard larger-particle phosphors in thin screens results in grainy images, poor resolution and low X-ray absorption. It is therefore necessary to significantly reduce the phosphor particle size and/or reduce light scattering to improve image resolution.
It has been determined that the highest resolution and lowest light scattering could be achieved only if the phosphors are disposed in microchannels. When microchannel substrates are filled with phosphors, a new class of high resolution microchannel phosphor screens become available for various medical imaging applications. By proper selection of the phosphors and substrate materials, the X-ray generated light propagates in a waveguide mode by means of internal reflection, thereby significantly reducing scattering. Thus, the microchannel screen of the present invention can dramatically enhance contrast and resolution and ensure more accurate detection and better diagnostic imaging capabilities
Our previous work in the design and construction of microchannel based X-ray screens can be found in U.S. Pat. No. 5,952,665; issued Sep. 14, 1999 Entitled Composite Nanophosphor Screen for Detecting Radiation”; U.S. patent application Ser. No. 09/688,662 filed Oct. 16, 2000 Entitled “High Resolution High Output Microchannel Based Radiation Sensor”; U.S. patent application Ser. No. 09/385,995 filed Aug. 30, 1999 Entitled “Microchannel High Resolution X-ray Sensor Having an Integrated Photomultiplier”, U.S. patent application Ser. No. 09/197,248 filed Nov. 20, 1998 Entitled “Composite Nanophosphor Screen For Detecting Radiation Having Optically Reflective Coatings”, U.S. patent application Ser. No. 09/688,662 filed Oct. 16, 2000 Entitled “High Resolution High Output Microchannel Based Radiation Sensor and PCT published application No. WO 99/28764. The disclosures of these previous US patent applications and issued patent are hereby incorporated by reference as if fully set forth herein.
Many processes for producing microchannel imaging plates are limited by the largest dimension of the plate that can be readily produced. Many otherwise suitable processes produce plates that have a maximum dimension of 2 to 4 inches. However, many X-ray imaging applications require plates of larger sizes. Larger size plates can be manufactured by combining a number of smaller plates by “tiling” or forming a mosaic of smaller plates. However, simply arranging smaller plates on a base substrate can make it very difficult to repair a damaged imaging screen even if only a small portion of the overall plate is damaged. The present invention provides an improved construction for imaging screens formed from a plurality of tiled microchannel plates.
Recently a new type of phosphor has been utilized in X-ray imaging screens, this phosphor is known as a “storage phosphor” or “photostimulable storage phosphor”. Screens comprised of storage phosphors may used in place of standard scintillator or film plates in X-ray imaging systems. After exposure to X-rays the storage phosphors will retain the X-ray image for a significant period of time so that the image can be read out at a remote location long after the image has been “exposed”. The image is read out by thermal or optical stimulation such as by scanning with a laser. After readout of the stored image, the imaging screen is then “reset” for subsequent reuse. These storage phosphor screens are very suitable for computerized radiography applications. However, previous storage phosphor imaging screens have not provided high resolution. The present application provides radiation imaging plates using storage phosphors of significantly increased resolution.
A storage phosphor imaging screen does not have to be read out in “real time”, thus such screens are particularly suitable for reading out from the side that is exposed to the X-ray rather than the other side. When a microchannel plate is read out from the same side that is exposed to X-ray (the “front” side), rather than from the opposite (“back”) side, a number of the design parameters of the microchannel plate are eased. The plate can be thicker, and thus easier to handle as only the phosphors in the upper portion of the plate are read out to form the image. In plates that are read from the back, overly thick plates means relatively long microchannels, which because of the many internal reflections of the light, can adversely affect light output with backside readout. The fact that only the phosphors in the upper portion of the microchannels are read also means the microchannel need not be filled all the way down and that the lower potion of the plate can be filled with non light emitting material. Furthermore, in a front read microchannel plate, the microchannels need also not be uniformly plated with a highly reflective coating all the way down the microchannels. The present application provides radiation imaging plates using storage phosphors disposed in microchannel plates that provide significantly increased resolution and are suitable for reading out from the front side.